Ion selective membranes, i.e., membranes having selectivity to ionic species, are an important class of permselective membranes, and are well known. They include anion exchange membranes, cation exchange membranes, and bipolar membranes. The primary objectives of those in the art of electrodialysis are the production of permselective membranes which exhibit high permselectivity and low resistance over long periods of time and which (in the case of bipolar membranes) have the ability to operate at high current densities. For these properties to be stable over an extended period of time, the membrane must, of course, be mechanically stable.
To date, a number of permselective membranes, including monopolar and bipolar membranes, have been produced which have a wide range of constructions and compositions and which, to at least some extent, meet the primary objectives outlined above. The notable monopolar membranes include Selemion.RTM. CMV cation membranes from Asahi Glass Co., Neosepta CL-25T of Tokuyama Soda Co., Ltd., and Dupont's Nafion.RTM. fluorocarbon membranes, especially Nafion.RTM. 110, 901 and 324 membranes. While these membranes have achieved some degree of commercial success, they are not acceptable for use in a number of applications. In the area of bipolar membranes, U.S. Pat. No. 4,116,889 discloses bipolar membranes which have achieved the basic goals essential for commercial applications. Nevertheless, further improvement in the performance of bipolar membranes is warranted, especially in the use of larger, e.g. 1 m.sup.2 surface area, bipolar membranes where mechanical stability is of great concern.